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Alum

Potassium aluminum sulfate is the mordant most frequently used by dyers for animal and cellulose fibers and fabrics. It improves light and wash-fastness of all natural dyes and keeps colours clear. It is inexpensive and safe to use (but should not be ingested). This form of alum is refined from bauxite, the raw state of aluminum ore, and is free from the impurities some other alums can contain (such as iron).

Aluminum acetate is sometimes used as the preferred alum mordant for cellulose fibres and fabrics. It is refined from bauxite, and acetic acid is used as a purifying agent. For this reason some natural dyes develop to a richer shade on cellulose.